To understand the substance of this book, you need a modest knowledge of what TCP/IP is and what it does. You’ll find more than enough information in Craig Hunt and Robert Bruce Thompson’s books on TCP/IP,[6] but what follows is, we think, what is necessary to know for our book’s purposes.
TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of protocols enabling computers to talk to each other over networks. The two protocols that give the suite its name are among the most important, but there are many others, and we shall meet some of them later. These protocols are embodied in programs on your computer written by someone or other; it doesn’t much matter who. TCP/IP seems unusual among computer standards in that the programs that implement it actually work, and their authors have not tried too much to improve on the original conceptions.
TCP/IP is generally only used where there is a network.[7] Each computer on a network that wants to use TCP/IP has an IP address , for example, 192.168.123.1.
There are four parts in the address, separated by periods. Each part corresponds to a byte, so the whole address is four bytes long. You will, in consequence, seldom see any of the parts outside the range 0 -255.
Although not required by the protocol, by convention there is a dividing line somewhere inside this number: to the left is the network number and to the right, the host number. Two machines on the same physical network — usually a local area network (LAN) — normally have the same network number and communicate directly using TCP/IP.
How do we know where the dividing line is between network number and host number? The default dividing line used to be determined by the first of the four numbers, but a shortage of addresses required a change to the use of subnet masks . These allow us to further subdivide the network by using more of the bits for the network number and less for the host number. Their correct use is rather technical, so we leave it to the routing experts. (You should not need to know the details of how this works in order to run a host, because the numbers you deal with are assigned to you by your network administrator or are just facts of the Internet.)
Now we can think about how two machines with IP addresses X and Y talk to each other. If X and Y are on the same network and are correctly configured so that they have the same network number and different host numbers, they should be able to fire up TCP/IP and send packets to each other down their local, physical network without any further ado.
If the network numbers are not the same, the packets are sent to a router , a special machine able to find out where the other machine is and deliver the packets to it. This communication may be over the Internet or might occur on your wide area network (WAN). There are several ways computers use IP to communicate. These are two of them:
A way to send a single packet from one machine to another. It does not guarantee delivery, and there is no acknowledgment of receipt. DNS uses UDP, as do other applications that manage their own datagrams. Apache doesn’t use UDP.
A way to establish communications between two computers. It reliably delivers messages of any size in the order they are sent. This is a better protocol for our purposes.
[6] Windows NT TCP/IP Network Administration, by Craig Hunt and Robert Bruce Thompson (O’Reilly & Associates, 1998), and TCP/IP Network Administration, Third Edition, by Craig Hunt (O’Reilly & Associates, 2002).
[7] In the minimal case we could have two programs running on the same computer talking to each other via TCP/IP — the network is “virtual”.